99% of Proxies are blocked at our school, and adding an 's' to the end of the 'http' only works on a FEW websites - but not any game or video based ones.
And our teachers also have a program to monitor your screen, and they can also remotely control your computer - I've had things of mine open, then my mouse starts moving on the screen and exits it by pressing the 'x'. Our history is also closely monitored by the School.
But I mean, HOW desperate are you to play Runescape at school?!?!?! Just wait until home - School is for Schoolwork.